Everything, but mostly that it gets its name based on what annoys others instead of what bothers us. Attention problems and Hiperactivity are just two tiny parts of ADHD. There are other much more significant symptoms
In general the disorder is related to not properly processing neurotransmitters so everything that is "managed" by neurotransmitters can be out of whack. And some folks seem to have more problems with one kind of neurotransmitters than others.
Neurotransmitters are things like Dopamine, Serotonin, Endorfine, Noradrenalin. Example of stuff that are managed by them: Movement, control of the body, stress, sleep, attention, memory, learning, inhibition, joy, pain relief.
So, just by that you can probably imagine how broad the effects of ADHD might be.
We still don't know any way to treat the root cause effectively (neurotransmitters being "killed"). The only thing that helps, is forcing the body to generate more of those neurotransmitters, hoping that it'll process more of them that way. That works even with different stuff. If we generate more Dopamine, the body ends up processing more of the Serotonin it already produces too. That's why stimulants work so well at regulating us - it floods our brain with artificial stuff that end up "shielding" the natural stuff to let them do their job too.
That is also why stimulants can sometimes make us more relaxed or even sleepy - it's not that the stimulant itself causes that, but it let's the body finally process everything properly so it can understand that it is supposed to be sleepy.
For someone without ADHD where the neurotransmitters are processed properly, stimulants will do nothing more than stimulate.
I've never had any support from others into managing my adhd so I can't say what helps for sure, but I can shed some light into it so you can try to find a way to help.
. 1. It's very hard for us to associate work and reward unless the reward is immediate. If you tell your kid "if you clean your room we can do X this weekend", they'll want to clean their room, but their "body" will still see it as a pointless chore.
. 2. "out of sight, out of mind". Imagine that people's brains are like an internet browser, with different stuff being in different tabs. For a NT person, there are a few tabs open with the stuff that they are doing that day and anything that is not relevant at the moment is saved on bookmarks to be retrieved at another time. The active tab is the thoughts that are currently going on in the head. For someone with ADHD, this browser would not have bookmarks and in turn it keeps the tabs open forever. As an effect of that, we can no longer manually switch between tabs. Once we switch to a different tab, the old one is lost and the only way to access it again is "clicking on a link to the same page". But we are so used to switching tabs all the time that everything loads instantly already.
Let me try to give practical examples of what I mean with this:
Say you live on the second floor of a building and you need to take the stairs to get home. Going up you notice the first step of the stairs is broken and need repairs. You make a note of it and continues going up. Thats a thought for the "stairs" tab that is currently active. You go into your house and notice your pet's food bowl. The browser now switches to the "feed pet" tab, which makes you realize you haven't done it that day yet. Anything about the stairs is now completely wiped from your head, as if you had never even thought about it. You go feed your pet and on the way you notice a pile of dirty clothes to wash. Your brain now switches to laundry tab and you forget anything about the pet. You start the laundry and go back to your living room, see the pet's food bowl again and goes "oh yeah I need to feed it" - this puts the pet tab back into your head. This time you carry the bowl with you so it keeps that tab active and you can complete the task. At night you're watching some show, commercial break hits and an ad shows someone going up some stairs so you go "fuck, the stairs" but it's night now and you can't do anything about it. Your wife comes in and asks what are you watching. You have no idea because you're on the "stairs" tab now. Commercial break ends, you see one character and that puts you back on the show tab, so you instantly remember the name and the whole plot.
If you expect someone with ADHD to do something, there's only a few ways they'll actually do it:
That third one is what we've come to call "waiting mode". It's what we do when we have an appointment at a specific time of the day for example. We hold on to that "tab" so hard to ensure we don't lose it, that we basically become unable to do anything else until that is done. When we're in waiting mode, simply looking at a clock will switch the active tab back to that appointment and make us lose track of whatever else we were trying to do. Everybody eventually develops this skill (sacrificing their whole day so they don't forget their appointment) after missing too many things - so don't expect your kid to be able to remember to do things on their own.
. 3. Living like this is tiring. Feeling like we have no control over where our own thoughts go. It's like there are bees inside our head constantly buzzing buzzing. And then at one point you find something that makes the bees sleep. Playing videogames, drawing, solving some logic puzzles - what it is changes for everyone, but your kid will find hobbies that will make the buzzing stop. Such a hobby will give great relief, on top of anything else a hobby gives us. But when the bees are sleeping, we are "frozen" into that tab - if left to our own devices we'll often forget to eat, sleep and everything else. Initially you'll have to ensure your kid doesn't get stuck on their hobby alone. Do remember though that everytime you take your kid off of their hobby, you're waking up the bees in their head. You may notice that their immediate reaction to it might be to be very annoyed. You'll both have to learn to manage it, but what I recommend is trying to keep interruptions to a minimum. If the kid needs to do things, try to get them to do them all at once so they can have more ininterrupted time too. If you wake the bees every 10 minutes, it can be infuriating.
. 4. Any relief that we get from doing rewarding things or from "putting the bees to sleep" are also contained to that "tab". If your kid spends a whole afternoon resting they'll feel rested during that afternoon, but as soon as you ask them to do some chore, it's as if they hadn't rested at all. Imagine like you had a clone of yourself and you have your clone do everything you don't like doing. It's kinda like that, but instead of being two different beings, your kid is switching between being the one that only rests and the one that only works. Doing the same chores every day feels more and more annoying every time we do it.
. 5. Kinda repeating one of my previous posts, but anything that is stashed away somewhere will eventually be forgotten. Things that are kept in plain sight will naturally see more use. Things may end up being suddenly forgotten too. For example if the kid is learning to play guitar and they practice every day for months, then one day they don't and it goes on for six weeks before they even remember they were learning the guitar, at which point the habit is completely broken. Habits in general are harder to form and once formed, we still need to put effort into keeping it or it may just vanish.
I could still write a lot more, but I should get going now, writing this made the bees sleep and I forgot to go to work.